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School Acoustics - Classroom Acoustics

School Acoustics - Classroom Acoustics

The quality of education that a child/person receives will be severely affected in classrooms with poor acoustics. A student cannot learn efficiently if he, or she, cannot hear the teacher clearly. Equally a teacher that has to shout to be heard will not only run the risk of vocal strain and other throat problems but can also damage the relationship they would otherwise build with the students. Teaching spaces that are very loud may also run the risk of placing the occupants in breach of the Noise at Work Regulations as their hearing may get damaged with prolonged exposure to high noise levels.

A large number of classrooms in the UK currently suffer from very poor acoustics. The main problems are due to:

excessive reverberation

noise transfer between rooms

external noise break-in

high levels of internal ambient noise

poor speech intelligibility

high levels of internal ambient noise

high levels of rain noise on roofs

badly designed open plan teaching spaces

low signal to noise ratios.

The Basics

When a soundwave strikes a surface three things generally happen. Some of the sound will pass through the surface into the adjacent space. This is referred to as Transmission. Some of the sound will be absorbed by the surface. This is referred to as Absorption. Some of the sound will reflect back into the space. This is referred to as Reflection. A fourth possibility is that some of the sound will be diffused by irregular shaped objects. Diffusion is where, rather than a single discrete reflection, the soundwave is scattered in many directions. This is referred to as Diffusion.

As a result, the reflected soundwave will not have the same amount of energy as the initial wave. This transfer of sound is frequency dependent i.e. high frequencies are more easily absorbed and low frequencies are more easily reflected. Different surfaces have what is called an Absorption Coefficient which is a measure of how much of the incident soundwave is absorbed against how much is reflected back into the space. In an ideal world an absorption coefficient of 1.00 means that 100% of the incident soundwave is absorbed by the material and nothing is reflected.

Reverberation Time

This is a measure of how quickly sound decays (from loud to quiet) in a room. It depends on the room volume and the type of surfaces. Generally the larger the volume of the room the longer the reverberation time, and the more reflective the surfaces the longer the reverberation time.

Long reverberation times are the main cause of poor classroom acoustics. According to BB93 an ideal classroom would have a reverberation time of less than 0.4 seconds for hearing impaired students and up to around 0.8 seconds for general purpose teaching spaces. In the UK many classrooms have reverberation times of more than 1.0 seconds

As mentioned above, the reverberation time can be reduced by either decreasing the volume or increasing the absorption. If the classroom is of a Victorian type with very high ceilings then it is sometimes possible to install an acoustically absorbent dropped ceiling. This not only reduces the effective volume of the space but also introduces additional absorption material over a large area. This solution may not work with high Victorian windows and it may be necessary to install new light fittings, smoke detectors and fire alarms etc.

Adding more efficient absorbent material to walls, floors and ceilings is another solution. For this sort of acoustic treatment to work efficiently the material needs to be spread evenly about the space (rather than just fixed to one wall). If, for example, treatment is only applied over the entire ceiling then the reverberation time may be decreased to an acceptable level. However this will not address the problems of reflections and flutter echoes from opposing wall surfaces. A carpet will only slightly improve things and is best for reducing the scraping noise of chairs.

Sound Transmission Loss

The sound transmission loss or noise reduction of a wall or floor between rooms is a measure of the amount of sound that passes through a partition. In simplistic terms it is calculated by subtracting the noise level in the ´receiving´ room from the noise level in the ´source´ room after normalising the reverberation in the receiving room to 0.5 seconds and weighting the results according the BS EN ISO 717.

Speech Intelligibility

This is a measure of how clearly words can be heard in a room

Signal to Noise Ratio

This is directly related to speech intelligibility in that it gives an estimate of how understandable speech is likely to be in a room. It is the ratio of the background noise level in dB subtracted from the sound level of the teacher's voice in dB. The higher the ratio the better the speech intelligibility. If the background noise is louder than the teacher's voice then speech will be extremely hard to understand especially at the back of the classroom. Generally a signal to noise ratio of at least +10dB gives good speech intelligibility, although children with a hearing impairment need +15dB. Less than +10dB significantly degrades speech intelligibility for hearing impaired children.

Reflections

Reflections from surfaces that are far apart and arrive at the listener after about 50ms are interpreted by the brain as discrete echoes and these interfere with speech intelligibility. Reflections arriving before 50ms act to reinforce the speech and are more beneficial.

Care needs to be taken when designing classrooms because too much absorption placed on a rear wall will reduce the voice energy being reflected back to the front of the room. Diffusing elements are sometimes more useful as they scatter the sound in many directions which means that the level in any one particular direction is reduced.

In some cases specific reflections can be beneficial. For example a classroom with a highly absorptive ceiling will greatly reduce the sound energy available at the back, and possibly sides, of the classroom.

HVAC Ambient Noise

High ambient noise from heating and ventilation systems can be a big problem and is usually down to poor initial design. Common causes of HVAC noise are:

air velocities being too fast

ducts being too narrow

air velocities being too fast

fan noise traveling down unlined ductwork

lack of, or incorrectly positioned, silencers

air velocities being too fast

incorrect design of shared supply and return duct paths.

Interesting Statistics

70% of deaf children are currently educated in mainstream schools

16-18% of children in schools have some sort of hearing difficulty

In a classroom with poor acoustics, young children understand only 36% of what is being said when 8m away from the teacher

Teachers spend 60% of their day using their voice

80% of teachers report vocal strain and other throat problems

86% of teachers feel external noise causes them problems

49% of teachers have to strain their voices to be heard

Teachers are 32% more likely to have voice problems compared to other professions

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